MOSFET protection using resistor-capacitor thermal network

ABSTRACT

A circuit for protecting a semiconductor element is provided in a system for supplying power from an input node to an output node. The circuit has an analog multiplier responsive to a voltage across the semiconductor element and a current flowing through the semiconductor element to produce an output voltage. A transconductance amplifier is coupled to an output of the analog multiplier for receiving the output voltage of the analog multiplier to produce an output current. An analog RC circuit coupled to the output of the transconductance amplifier is configurable to include a selected number of resistive elements having selected resistance values and a selected number of capacitive elements having selected capacitance values. The configuration of the RC circuit is carried out to provide an RC thermal model that reproduces a desired thermal behavior of the semiconductor element. The RC circuit is responsive to the output current of the transconductance amplifier to produce an output voltage used to control the semiconductor element.

RELATED APPLICATIONS

This application is a Divisional Application of U.S. Ser. No.14/486,697, filed Sep. 15, 2015, which claims priority of U.S.provisional application No. 61/900,832, filed on Nov. 6, 2013. Thesubject matter of each is incorporated herein by reference in entirety.

TECHNICAL FIELD

This disclosure relates to circuits for protecting semiconductorelements in electrical systems. In particular, the disclosure presentsways to protect semiconductor elements, such as MOSFETs, from excessiveheating.

BACKGROUND ART

A hot swap circuit applies power from an input source to a load in acontrolled and protected fashion. One function of such a controller isto limit inrush currents from the power source to the load, especiallyload capacitance, when power is first applied or if the power sourcevoltage suddenly increases. Another function is to limit current if theload attempts to draw too much current, for example if there is a shortcircuit in the load.

FIG. 1 shows a hot swap circuit that uses a single MOSFET 100 (Q1) inseries with a current sense resistor 102 along with control circuitryfor limiting current. Numerous such circuits are commercially available.When limiting current, a current limit amplifier 106 adjusts the MOSFETgate to source voltage in order to limit the voltage across the currentsense resistor 102 and thus the current through the MOSFET 100. Thecurrent limit amplifier 106 compares a voltage representing the currentin the current sense resistor 102 with a voltage VLIMIT produced by avoltage source 104 to control the gate of the MOSFET 100 so as to reducethe output current when the sensed current exceeds a maximum valueestablished by the voltage VLIMIT. A current source 108 is provided forpulling up the gate voltage. A transistor 110 is provided for turningthe hot swap circuit on or off.

During this time, the voltage and current through the MOSFET can both belarge, resulting in high power dissipation in the MOSFET. If this powerdissipation persists, the MOSFET can reach temperatures that causedamage. MOSFET manufacturers present the safe limits on MOSFET voltage,current, and time as a curve referred to as the Safe Operating Area(SOA). Commonly, a timer circuit sets a maximum time for the MOSFET tooperate in a current limit mode. When this time expires, the MOSFET isturned off to protect it from overheating. The load will lose power andthe hot swap controller will indicate that a fault has occurred. Thetimer circuit may include a timer capacitor C_(TIMER) coupled to a 2 μAcurrent source 112, which via a switch S1 is coupled to a 100 μA currentsource 114. The switch S1 is controlled by a control signal produced atthe status node of the current limit amplifier 106 that indicateswhenever the current limit amplifier 106 limits the current.

Often high power hot swap applications need to charge large bypasscapacitors 126 (CL) across the load. To reduce stress on the MOSFET 100,the load may be kept off until the bypass capacitors 126 are charged. Asmall charging current for the capacitance keeps the power in the MOSFET100 low enough to prevent a dangerous rise in temperature.

However, in the method described above, the timer runs at an equal rateany time the circuit is in a current limit mode. The timer time-out, ata minimum, must be set to allow the circuit to completely charge thebypass capacitor 126 from ground. An even longer time-out setting may berequired if another allowable operating condition, such as a fastincrease in the input supply voltage or the presence of a load currentduring start-up, causes an even longer duration current-limit event. AMOSFET 100 must be selected that can withstand the worst-case SOAcondition that occurs during any possible normal operating condition orfault condition. Fault conditions may include events such as start-upinto a short circuit that will result in the entire supply voltage beingimposed across the MOSFET 100 for the time-out duration. This is a faultcondition that requires a greater SOA of the MOSFET 100 than any normaloperating condition.

With the previously described method, the worst-case SOA conditionoccurs during a fault condition, and the customer must select a MOSFET100 that survives this condition. The worst-case condition is not alwaysreadily apparent, and determining the worst-case condition is sometimesthe most challenging aspect of designing a hot swap circuit.

Therefore, there is a need for circuit and methodology for MOSFETprotection that would overcome the disadvantages discussed above.

SUMMARY OF THE DISCLOSURE

The present disclosure offers a circuit for protecting a semiconductorelement in a system for supplying power from an input node to an outputnode. The circuit comprises an analog multiplier responsive to a voltageacross the semiconductor element and a current flowing through thesemiconductor element to produce an output voltage. A transconductanceamplifier is coupled to an output of the analog multiplier for receivingthe output voltage of the analog multiplier to produce an outputcurrent. An analog RC circuit coupled to the output of thetransconductance amplifier is configurable to include a selected numberof resistive elements having selected resistance values and a selectednumber of capacitive elements having selected capacitance values. Theconfiguration of the RC circuit is carried out to provide an RC thermalmodel that reproduces a desired thermal behavior of the semiconductorelement.

The RC circuit is responsive to the output current of thetransconductance amplifier to produce an output voltage. A comparatorcompares the output voltage of the RC circuit with a reference voltageto produce a control signal supplied to the semiconductor element.

In accordance with one aspect of the disclosure, the semiconductorelement may include a MOSFET. A sense resistor may be coupled to theMOSFET for sensing the MOSFET current.

A first input of the analog multiplier may be configured for receiving avoltage across the sense resistor, and a second input of the analogmultiplier may be configured for receiving a voltage between source anddrain terminals of the MOSFET.

For example, the MOSFET may be arranged in a hot swap circuit.

Also, the MOSFET may be configured in a surge stopper circuit. A surgestopper circuit contains all of the functionality of a hot swap circuit,and adds the ability to limit the output voltage to a maximum value. Thesurge stopper circuit may include a resistor divider coupled to theoutput node, and a feedback node provided in a common node betweenresistive elements of the resistor divider. If the output voltage of thesurge stopper circuit rises to a level that causes the feedback node toreach a reference voltage, a voltage limit amplifier drives the gate ofthe MOSFET to regulate the feedback node at the reference voltage. Aswitch is turned on, enabling the TIMER pullup current, when the currentlimit amplifier is active and limiting the MOSFET current or the voltagelimit amplifier is active and limiting the output voltage. The TIMERpullup current limits the amount of time that the current limit orvoltage limit is active to protect the MOSFET.

The output current of the transconductance amplifier in the hot swap andsurge stopper circuits is proportional to the power dissipated by theMOSFET.

If the MOSFET current is limited to a fixed maximum value, which iscommon in hot swap and surge stopper circuits, the voltage across theMOSFET's drain and source is indicative of the power dissipation of theMOSFET. When the current is below the maximum value, the voltage betweenthe drain and source is negligible and the MOSFET's power dissipation isnegligible. As a result, when the MOSFET current is limited to a fixedmaximum value, the analog multiplier may be implemented by a circuitthat produces an output voltage that is proportional to the voltageacross the MOSFET's drain and source.

For example, the RC circuit may represent a Cauer thermal model or aFoster thermal model. More generally, it may be formed of any network ofresistors and capacitors that produces an output voltage proportional tothe MOSFET silicon junction temperature when a current proportional topower is input to the network.

In accordance with a further aspect, the present disclosure provides amethod of protecting a MOSFET in a system for supplying power from aninput node to an output node. The method involves configuring an analogRC circuit to represent a thermal model of the MOSFET reproducing adesired thermal behavior of the MOSFET.

Using an analog multiplier, the power dissipation of the MOSFET may bedetermined as a product of a MOSFET current and a voltage across theMOSFET. Further, a current proportional to the power dissipation of theMOSFET may be produced and supplied to the RC circuit. A voltageproduced by the RC circuit in response to the produced current iscompared with a reference value to control switching of the MOSFET.

The analog RC circuit may be configured by including a selected numberof resistive elements and a selected number of capacitive elements, andselecting desired resistance values of the resistive elements anddesired capacitance values of the capacitive elements.

An additional implementation may include a method to detect the casetemperature of the MOSFET. This may reduce the number of resistor andcapacitor elements required in the RC thermal model because only thetemperature difference between the silicon junction and the casetemperature of the MOSFET is modeled. Typically, the time constant fromthe silicon junction to the case of the MOSFET is between 10 ms and 1second. If the MOSFET case temperature is not sensed, it may benecessary to include the board temperature behavior in the RC thermalmodel. This time constant may be several minutes. Modeling both shortand long times accurately with a single thermal model requires moreresistors and capacitors than representing only shorter times or longertimes accurately. Additionally, when modeling both short and long timeconstants in a single thermal model, the analog multiplier andtransconductance amplifier must be accurate over a wide range of powerinputs which may be difficult to design. This temperature sensingbehavior may be implemented using a resistive divider where the topresistor has a positive temperature coefficient and is electricallyconnected to the MOSFET drain and is in close proximity on the board.Resistors with accurately specified temperature coefficients are widelyavailable at relatively low cost. This results in a circuit that crossesa threshold (and results in the MOSFET turning off) when the MOSFET casetemperature rises above a predetermined temperature.

In another implementation, the case temperature may be sensed by anelement such as a diode-connected NPN transistor. A voltage that isproportional to the MOSFET case temperature is output at a temperaturesensing node connected to a node in the thermal network that representscase temperature. For example, in the Cauer network, the temperaturesensing node is connected in series with the last resistor in thenetwork.

Additional advantages and aspects of the disclosure will become readilyapparent to those skilled in the art from the following detaileddescription, wherein embodiments of the present disclosure are shown anddescribed, simply by way of illustration of the best mode contemplatedfor practicing the present disclosure. As will be described, thedisclosure is capable of other and different embodiments, and itsseveral details are susceptible of modification in various obviousrespects, all without departing from the spirit of the disclosure.Accordingly, the drawings and description are to be regarded asillustrative in nature, and not as limitative.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description of the embodiments of the presentdisclosure can best be understood when read in conjunction with thefollowing drawings, in which the features are not necessarily drawn toscale but rather are drawn as to best illustrate the pertinent features,wherein:

FIG. 1 illustrates a conventional hot swap controller.

FIG. 2 presents an example of a safe operating area (SOA) plot.

FIG. 3 illustrates an example of a transient thermal impedance plot.

FIG. 4 shows an exemplary embodiment of a protection circuit forprotecting a MOSFET in a hot swap controller.

FIG. 5 shows an exemplary embodiment of a protection circuit forprotecting a MOSFET in a surge stopper circuit.

FIG. 6 illustrates an exemplary embodiment of a protection circuit withMOSFET case temperature sensing for protecting a MOSFET in a hot swapcontroller.

FIG. 7 shows an exemplary embodiment of a protection circuit with MOSFETcase temperature sensing and producing a temperature dependent voltageat a terminal of an RC thermal network in a surge stopper circuit.

DETAILED DISCLOSURE OF THE EMBODIMENTS

The present disclosure provides a technique for ensuring that the safeoperating area (SOA) of a MOSFET is not exceeded. Historically, thecircuit designer has relied on a “safe operating area” (SOA) plot tojudge the capability of the MOSFET. An example of the SOA plotreproduced from the Vishay IRF530 datasheet is illustrated in FIG. 2.Cost and physical size of a MOSFET are generally proportional to its SOAcapability, so a designer would like to minimize the required safeoperating area while safely powering up the output or riding throughmomentary overload events.

MOSFET safe operating area is limited by the maximum allowable junctiontemperature of the MOSFET die. In FIG. 2, the maximum junctiontemperature is specified as 175° C. when the case temperature is held at25° C.

The SOA plot is usually derived from a MOSFET's “transient thermalimpedance”. FIG. 3 shows an example of the transient thermal impedanceplot reproduced from the Vishay IRF530 datasheet. Given the transientthermal impedance and the MOSFET power dissipation (versus time), onecan calculate the junction temperature rise.

FIG. 4 shows an exemplary embodiment of a MOSFET protection circuit ofthe present disclosure that protects a MOSFET 100 in a hot swap circuitsimilar to the circuit shown in FIG. 1 by replacing the conventionaltimer shown in FIG. 1 with a network of resistors and capacitors (RCnetwork) fed by a current proportional to MOSFET power dissipation. Thiscurrent is identified as I_(TIMER) current in FIG. 4.

For example, the RC network may include resistors R1, R2, R3 andcapacitors C1, C2, C3 arranged as shown in FIG. 4. When the voltageacross the RC network reaches a specified value V_(REF), the MOSFET 100is switched off. The I_(TIMER) current and the RC network represent ananalog model of the MOSFET thermal behavior in the electrical domain.The voltage at the node where the I_(TIMER) current is injected isproportional to the junction-to-case temperature rise. By configuringthe proper RC network, it is possible to control the hot swap circuit toturn off the MOSFET 100 automatically at the moment the MOSFET 100reaches its maximum junction temperature.

In the example shown in FIG. 4, the MOSFET protection circuit includesan analog multiplier 130 that multiplies the value of the MOSFET currentand the value of the voltage across the MOSFET to produce an outputvoltage. The value of the MOSFET current may be sensed by the senseresistor 102 (R_(S)) and may be provided to an input of the analogmultiplier 130 as a voltage supplied from nodes SENSE+ and SENSE− of thehot swap circuit. The MOSFET voltage value supplied to the other inputof the analog multiplier 130 may be represented by a voltage betweendrain and source terminals of the MOSFET 100 sensed across nodes SENSE−and SOURCE of the hot swap circuit.

The output voltage of the analog multiplier 130 is supplied todifferential inputs of a transconductance amplifier 132 that producesthe I_(TIMER) current proportional to power dissipation of the MOSFET100.

Alternatively, assuming the voltage across the MOSFET 100 is significantonly when the circuit is in a current limit mode, the I_(TIMER) currentmay be produced by the transconductance amplifier 132 with its inputvoltage proportional to the voltage observed across the MOSFET 100.

The I_(TIMER) current flows through the RC network that produces avoltage compared by a comparator 134 with a predetermined referencevoltage Vref. When the voltage produced by the RC network reaches thereference voltage Vref, the MOSFET 100 is turned off. For example, theoutput signal of the comparator 134 may be supplied to the gate of theMOSFET 110 to turn off MOSFET 100.

The RC network may be configured to represent an electric analog modelof a desired thermal behavior associated with any MOSFET. In particular,the configuration of the RC network for a particular MOSFET may involveselection of a desired number of resistive and capacitive elements inthe RC network and selection of their resistance and capacitance values.The RC network may be configured to represent any desired RC thermalmodel such as a Foster model or Cauer model.

Hence, to provide an appropriate protection of a MOSFET, the customer isonly required to select the MOSFET that satisfies the SOA required fornormal operating conditions. The MOSFET is automatically turned offbefore it is subjected to any condition that would exceed its SOArating.

FIG. 5 illustrates an exemplary embodiment of a MOSFET protectioncircuit that protects MOSFETs in a surge stopper circuit that implementsthe hot swap functionality described above in connection with FIG. 1,but additionally limits an output voltage Vout to a maximum value whichmay be configured with a user selectable resistor divider Ro1, Ro2.Alternatively, the output voltage may be fixed internally inside theintegrated circuit.

In addition to elements shown in FIG. 1, the surge stopper circuit inFIG. 5 includes a resistor divider Ro1 and Ro2 coupled to the outputterminal Vout. A common node between the resistors Ro1 and Ro2 iscoupled to a feedback node FB of the surge stopper circuit.

The MOSFET protection circuit in FIG. 5 includes a configurable RCcircuit having resistive elements R1, R2, R3 and capacitive elements C1,C2, C3, a transconductance amplifier 140 for producing current I_(TIMER)supplied to the RC circuit, an analog multiplier 142 coupled todifferential inputs of the transconductance amplifier and an amplifier144. The output of the current limit amplifier 106 is coupled to thegate of the MOSFET 100 via a diode D1, and the output of the amplifier144 is coupled to the gate of the MOSFET 100 via a diode D2.

The analog multiplier 142 multiplies the value of the MOSFET currentsensed by the sense resistor 102 and the value of the voltage across theMOSFET 100 to produce an output voltage. The value of the MOSFET currentmay be provided to an input of the analog multiplier 142 as a voltagesupplied from nodes SENSE+ and SENSE− of the surge stopper circuit. TheMOSFET voltage value supplied to the other input of the analogmultiplier 142 may be represented by a voltage between drain and sourceterminals of the MOSFET 100 sensed across nodes SENSE− and SOURCE of thesurge stopper circuit.

Alternatively, assuming the voltage across the MOSFET 100 is significantonly when the circuit is in a current limit mode, the I_(TIMER) currentmay be produced by the transconductance amplifier 140 with its inputvoltage proportional to the voltage observed across the MOSFET 100. Thisalternative implementation is possible because at those times when thesurge stopper circuit is not in a current limit mode, the voltage acrossMOSFET 100 is negligible and the I_(TIMER) current is substantially at azero level. When the surge stopper circuit is in a current limit mode,the I_(TIMER) current is proportional to the voltage across the MOSFET100. Because the current limit operation forces the MOSFET current to afixed value, the resulting I_(TIMER) current is therefore proportionalto the power across the MOSFET 100. When the voltage at the RC networkreaches a prescribed value, the MOSFET 100 is turned off.

Similar to the circuit in FIG. 4, the RC circuit in FIG. 5 can beconfigured to reproduce any desired thermal behavior of the MOSFET 100.In particular, any selected number of resistive and capacitive elementshaving any selected resistance and capacitance values may be included inthe RC circuit to simulate a thermal behavior of the MOSFET 100. The RCcircuit in FIG. 5 may be configured to represent any desired RC thermalmodel such a Foster model or a Cauer model.

FIG. 6 illustrates a MOSFET protection circuit of the present disclosurethat protects a MOSFET 100 in a hot swap circuit. This protectioncircuit is similar to the circuit in FIG. 4, but adds to this circuitthe ability to turn off the MOSFET 100 when the case temperature ofMOSFET 100 exceeds a predetermined threshold. A resistive dividercomposed of resistors R_(T1) and R_(T2) is connected between the MOSFETdrain and ground and one or both of these resistors have a specifiedtemperature coefficient. For example, in FIG. 6, it is assumed that theresistor R_(T1) connected to the MOSFET drain has a positive temperaturecoefficient and the resistor R_(T2) connected to ground has a negligibletemperature coefficient. In most commercially available MOSFETs, thedrain is the portion of the case with the best thermal connection to theMOSFET silicon die. If another portion of the MOSFET case (such as thesource terminal) has a better thermal connection to the silicon die, thetop resistor alternatively could be connected to that terminal instead.The MOSFET case temperature threshold is detected when the voltage atthe common node TEMP of the resistive divider crosses a predefinedvoltage V_(REF1) indicating that the case temperature of the MOSFET 100exceeds a predetermined threshold.

In particular, a comparator 136 may have an inverting input coupled tothe TEMP node and a non-inverting input supplied with the V_(REF1)voltage. The output of the comparator 136 is connected to an input of anOR gate circuit 138, the other input of which is connected to the outputof the comparator 134. When either the voltage produced by the RCnetwork reaches the reference voltage Vref, or the voltage at the TEMPnode exceeds the V_(REF1) voltage, the output of the OR gate 138produces a signal supplied to the gate of the MOSFET 110 to turn off theMOSFET 100.

FIG. 7 illustrates a MOSFET protection circuit that protects MOSFETs ina surge stopper circuit. This circuit is similar to the circuit in FIG.5, but adds to this circuit the ability to sense the case temperature ofthe MOSFET and produce a voltage at one terminal of the RC thermalnetwork.

In the example shown in FIG. 7, a diode-connected NPN transistor 146 isthermally coupled to the MOSFET 100 to sense the case temperature of theMOSFET 100. For example, the NPN transistor 146 may be arranged on a PCboard in close proximity to the MOSFET 100. Alternatively, if the MOSFET100 is mounted to a heatsink, the NPN transistor 146 may be thermallyconnected to and electrically isolated from the heatsink using aheatsink pad made from a material such as silicone elastomer or mica.

The emitter of the transistor 146 is grounded. The base and collector ofthe transistor 146 via a node VBE is coupled to a temperature sensor 148which produces at a node TEMP a voltage proportional to the casetemperature of the MOSFET 100. The TEMP node is connected to a terminalof the RC thermal network, and the output of this network is connectedto the TIMER node. By producing a voltage proportional to the MOSFETcase temperature at the TIMER terminal of the RC thermal network, it ispossible to reduce the number of resistors and capacitors necessary toaccurately produce a voltage at the TEMP node that is proportional tothe silicon die temperature over both short and long time durations.

The I_(TIMER) current flows through the RC thermal network that producesa voltage compared by a comparator 150 with a predetermined referencevoltage V_(REF2). When the voltage produced by the RC network reachesthe reference voltage V_(REF2), the output signal of the comparator 150supplied to the gate of the MOSFET 110 turns off the MOSFET 100.

The MOSFET protection of the present disclosure can also be applied tolinear regulators. Like surge stoppers, linear regulators also limit theoutput voltage to a maximum value which may be configured with a userselectable resistor divider or fixed internally inside the integratedcircuit.

The foregoing description illustrates and describes aspects of thepresent invention. Additionally, the disclosure shows and describes onlypreferred embodiments, but as aforementioned, it is to be understoodthat the invention is capable of use in various other combinations,modifications, and environments and is capable of changes ormodifications within the scope of the inventive concept as expressedherein, commensurate with the above teachings, and/or the skill orknowledge of the relevant art.

The embodiments described hereinabove are further intended to explainbest modes known of practicing the invention and to enable othersskilled in the art to utilize the invention in such, or other,embodiments and with the various modifications required by theparticular applications or uses of the invention. Accordingly, thedescription is not intended to limit the invention to the form disclosedherein.

What is claimed is:
 1. A system for supplying power from an input nodeto an output node, the system comprising surge stopper circuitryincluding a semiconductor element that includes a MOSFET in a case, thesurge stopper circuitry comprising: a circuit for protecting thesemiconductor element, comprising: an analog multiplier responsive to avoltage across the semiconductor element and a current flowing throughthe semiconductor element to produce an output voltage representing aproduct of values representing the voltage across the semiconductorelement and the current in the semiconductor element, a transconductanceamplifier coupled to an output of the analog multiplier for receivingthe output voltage of the analog multiplier to produce an outputcurrent, an analog RC circuit coupled to the output of thetransconductance amplifier and configurable to include a selected numberof resistive elements having selected resistance values and a selectednumber of capacitive elements having selected capacitance values, the RCcircuit being configurable to provide an RC thermal model thatreproduces a desired thermal behavior of the semiconductor element, theRC circuit being responsive to the output current of thetransconductance amplifier to produce an output voltage, and acomparator for comparing the output voltage of the RC circuit with areference voltage to produce a control signal supplied to thesemiconductor element, and a sensor that senses the temperature of theMOSFET case, wherein the circuit has a configuration that supplies avoltage that is proportional to case temperature to a node of the RCcircuit to produce a thermal model with the desired thermal behavior ofthe semiconductor element.
 2. The circuit of claim 1, wherein the MOSFETis coupled to a sense resistor configured for sensing the MOSFETcurrent.
 3. The circuit of claim 2, wherein a first input of the analogmultiplier is configured for receiving a voltage across the senseresistor.
 4. The circuit of claim 3, wherein a second input of theanalog multiplier is configured for receiving a voltage between sourceand drain terminals of the MOSFET.
 5. The circuit of claim 1, whereinthe output current of the transconductance amplifier represents powerdissipation of the semiconductor element.
 6. The circuit of claim 1,wherein the MOSFET current is limited to a fixed value, and the voltageacross the drain and source terminals of the MOSFET determines thecurrent at the input of the RC circuit.
 7. The circuit of claim 1,wherein the surge stopper circuit includes a resistor divider coupled tothe output node, and a feedback node provided in a common node betweenresistive elements of the resistor divider.
 8. The circuit of claim 1,wherein the RC circuit represents a Cauer thermal model.
 9. The circuitof claim 1, wherein the RC circuit represents a Foster thermal model.10. The circuit of claim 1, wherein a diode-connected transistor is usedto detect the MOSFET case temperature.
 11. The circuit of claim 10,wherein the voltage proportional to the MOSFET case temperature issupplied to a node coupled in series with a resistor of a Cauer thermalnetwork.